Organic impurity removal process for bayer liquors

ABSTRACT

A process for the removal of organic impurities in a Bayer process liquor in which wet and dry oxidation processes are linked together in a complementary way, such that the weaknesses of each individual organic removal process become a strength of the combined process. An organics-rich liquor stream is fed to a wet oxidation process to produce a first processed liquor which is depleted in organic compounds, but enriched in sodium carbonate and/or sodium oxalate. A feed slurry is fed to the dry oxidation process, typically including a liquor burner, where sodium compounds react to produce a discharge product. The first processed liquor and discharge product are directed into a leach tank, resulting in the formation of precipitated products including sodium carbonate and/or sodium oxalate. The precipitated products are separated from the leach tank and recycled in a mix tank providing the feed slurry to the liquor burner.

FIELD OF THE INVENTION

The present invention relates to a process for the removal of organicimpurities in a Bayer process liquor.

BACKGROUND TO THE INVENTION

The accumulation of organic impurities in Bayer process liquors is aproblem faced by most alumina refineries. The bulk of these impuritiesenter as contaminants within the bauxite, although a small proportionenters the liquor stream as a result of process additives such asflocculants and antifoams. Because Bayer liquors are highly caustic mostof the organic compounds hydrolyse and are therefore present as theirsodium salts. Caustic-insoluble organics generally depart with the mudresidue and play no further part in the Bayer process. Apart from thedirect deleterious effects of these organic species on the aluminarefinery's productivity and product quality, a proportion decomposes toform sodium carbonate and sodium oxalate. These latter contaminantscreate a variety of problems in their own right, including causticconsumption, reduced yield and degraded product quality. Consequently,most refineries already operate processes to control the levels ofsodium carbonate (by causticisation with slaked lime) and oxalate.However, far fewer refineries practice organic impurity removal, and themajor reason for this is that existing organic removal processes areeither complicated, expensive, or form side-products that are almost asproblematical as the organics themselves.

Most organic removal processes operate via some variation on theprincipal of oxidative destruction of the organics. These processes canbe performed either in the solid phase (calcination or liquor burningtechnologies) or in the aqueous phase (“wet oxidation” technology, inwhich the oxidation is effected either by chemical or electrical means).Both the “solid” (dry) and “liquid” (wet) phase processes suffer someserious disadvantages, which will be discussed below. Other organicremoval processes such as the use of liquid anion exchange resins,ultrafiltration, or adsorbent materials such as magnesite, are of nointerest in the present application, and will not be considered.

Two of the more commonly used dry organic destruction processes includeliquor burning and salting-out evaporation. In the former process,liquor is evaporated to dryness in contact with gibbsite or alumina toform pellets which are then calcined. The oxidation products, along withthe sodium carbonate and sodium hydroxide in the liquor react with thealumina to form sodium aluminate, which is subsequently dissolved andreturned to the process. Thus, the process “causticises” the organics,recovering the valuable soda. Unfortunately, the process is complicatedand energy-inefficient, most of the energy being consumed in evaporatingthe water from the feed liquors.

The process of salting-out evaporation is similar. In this case, aliquor stream is deeply evaporated, resulting in the “salting-out” ofimpurities, such as organic sodium salts, sodium oxalate, sodiumcarbonate and sodium sulphate. The solid impurities are separated fromthe supernatant liquor by filtration or centrifugation. The filtrate orcentrate is returned to the process, while the solids are eitherdisposed directly (resulting in a substantial loss of soda values),reacted with lime to causticise the sodium carbonate component, or mixedwith bauxite and fed to a kiln. In the kiln, the carbonate, oxalate andorganic species react with the bauxite to form mainly sodium aluminateand sodium ferrate. The kiln products are then reslurried and directedeither to the digestion circuit, or the clarification circuit of theBayer process. The salted-out solids are often very viscous and poorlycrystalline, and can be difficult to separate from the supernatantliquor. Like liquor burning, the process is also very energy-inefficientrequiring the evaporation of large quantities of water.

Wet oxidation processes involve reaction of the organic species with anoxidising agent. such as oxygen, ozone, chlorine or manganese dioxide.Contamination of the liquor stream, toxicity and reagent costs areprohibitive with most reagents other than oxygen. Oxidation using oxygenor ozone can be effective and economical, but requires operation atelevated temperatures and pressures for maximum efficiency. Safety is aserious concern with this process, as dangerous levels of hydrogen canbe evolved in these high temperature processes. Electrolytic processeshave been investigated at a laboratory level, but remain untried on apilot or plant scale.

All of the wet oxidation processes suffer from a serious disadvantage inthat they produce large quantities of sodium carbonate, and in mostcases, sodium oxalate. This places considerable strain upon therefinery's existing carbonate and oxalate removal facilities. Inpractice, this will usually necessitate the construction of additionalcausticisation and oxalate removal capacity, together with increasedconsumption of reagents such as lime.

SUMMARY OF THE INVENTION

The present invention was developed with a view to providing an improvedorganic removal process in which wet and dry organic removal processesare combined in a complementary manner such that the weaknesses of eachindividual process become a strength of the combined process.

According to one aspect of the present invention there is provided aprocess for the removal of organic impurities from a Bayer processliquor, the process including the steps of:

feeding a Bayer liquor stream rich with organic impurities to a wetoxidation process to produce a first processed liquor which is depletedin organic compounds, but enriched with sodium carbonate and/or sodiumoxalate;

reacting a substantial component of the sodium compounds in a feedslurry using a dry oxidation process to produce a processed dischargeproduct,

feeding at least a portion of the first processed liquor to a leach tankliquor to which is added the processed discharge product from the dryoxidation process, wherein the sodium carbonate and/or sodium oxalateprecipitate in the leach tank liquor; and,

separating the precipitated sodium carbonate and/or sodium oxalate fromthe leach tank liquor and recycling the precipitated products in thefeed slurry to the dry oxidation process;

whereby, in use, organic impurities in the Bayer liquor stream andresidual organic impurities remaining in the first processed liquor orin the recycled precipitated products, are causticised to sodiumaluminate or sodium ferrate in the dry oxidation process.

Preferably, substantially all of the Bayer liquor stream is fed to thewet oxidation process first and the balance (if any) of the firstprocessed liquor (that which is not fed to the leach tank liquor), isfed to the dry oxidation process.

Typically said balance of the first processed liquor and the recycledprecipitated products are fed to a mix tank for the dry oxidationprocess.

Typically the dry oxidation process employs a liquor burner. Optionallythe wet oxidation process also employs an evaporator. Advantageously theprocess of the present invention is combined with a sulphate removalprocess which is the subject of Australian patent No. 673306, thecontents of which are incorporated herein by reference.

In such an arrangement, a proportion of the processed discharge productfrom the dry oxidation process is fed to a second leach tank liquorhaving a caustic concentration sufficient to ensure the solubility ofgibbsite is not exceeded, and wherein the feed of said processeddischarge product to the second leach tank liquor is regulated to ensurethat the amount of sodium sulphate in said processed discharge productis substantially equal to the total input of sulphate to the process.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a more comprehensive understanding of the natureof the invention, preferred embodiments of the process for the removalof organic impurities will now be described in detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a simplified conceptual flow diagram illustrating theprinciple of a preferred process in accordance with the invention;

FIG. 2 is a conceptual flow diagram illustrating a variation of theprocess of FIG. 1, and,

FIG. 3 is a conceptual flow diagram illustrating a preferred embodimentof the process for the removal of organic impurities from a Bayerprocess liquor in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is based on the discovery that wet and dryoxidation processes may be linked together in a complementary way, suchthat the weaknesses of each individual organic removal process become astrength of the combined process. Typically the total organic removalcapacity of the combination is only slightly less than the sum of theindividual processes, but with substantially reduced energy consumptionand no undesirable by-products. The combined system may alternatively beviewed as a means of substantially increasing the organics destructioncapacity of the liquor burner, without requiring enlargement of theliquor burning plant.

The basic concept of a preferred process in accordance with theinvention is illustrated in the flow diagram of FIG. 1. In the preferredprocess, an organics-rich liquor stream 10, such as a spent Bayerliquor, is fed to both a wet oxidation process 12 and to the feed tank14 of a dry oxidation process 16. Throughout the present specification,a liquor burner 16 is used as the dry oxidation process, but any processwhich reacts sodium compounds with aluminium or iron oxides orhydroxides to produce solid sodium aluminate or sodium ferrates can beapplied. A liquor burning process is the subject of U.S. Pat. No.4,280,987 by Yamada et al, which corresponds to Australian Patent No.523,504. A refinement of the process is the subject of Australian patentapplication No. 70264/91. Any suitable wet oxidation process may beemployed, for example, the process described in GB2,037,722. Thecontents of these prior art patent specifications are incorporatedherein by reference.

As shown in FIG. 1, some of the organics-rich liquor stream is oxidisedwithin the wet oxidation process 12 to produce a first processed liquorwhich is depleted in organic sodium compounds, particularly humicsubstances, but enriched with sodium carbonate and/or sodium oxalate.This wet-oxidised processed liquor is directed to a leach tank 18immediately following the liquor burner 16.

In the process of FIG. 1, some of the organics-rich liquor stream is fedto the feed (mix) tank 14 for the liquor burner 16. A substantialcomponent of the organic impurities in the Bayer liquor stream isremoved in the liquor burner 16 and the liquor burner kiln dischargeproduct is directed into the leach tank 18, where it dissolves in theprocessed liquor from the wet oxidation process 12. The resultantincrease in the leach tank solution's ionic strength causes the sodiumcarbonate and sodium oxalate to become supersaturated, so that theyprecipitate from the leach tank liquor. Although the targetconcentration of the liquor in the leach tank 18 is limited only by thesolubility of sodium aluminate, in practice best performance is obtainedfrom caustic (‘C’) concentrations in the range of 200-600 g/l. The lowhumate content, and high carbonate and oxalate content of thewet-oxidised processed liquor contribute to the formation of aprecipitate that is easily separable from the supernatant liquor using afilter or centrifuge 20.

After solid/liquid separation, the precipitated solids are recycled tothe mix tank 14 as feed to the liquor burner 16. The concentratedsupernatant liquor is returned to the Bayer process. In the mix tank 14,the recycled sodium carbonate and sodium oxalate are reslurried in aminimal amount of spent liquor along with enough gibbsite or alumina toensure full causticisation in the liquor burner 16. As the feed to theliquor burner 16 is a dense slurry, no additional evaporation isrequired. Organics in the spent liquor, the recycled solids and anyresidual organics or sodium carbonate remaining in the wet oxidisedprocessed liquor adhering to the recycled precipitated solids, enter theliquor burner 16 and are causticised to sodium aluminate.

EXAMPLE 1

The benefits of the proposed system are illustrated in the followingexample. The performance of a typical conventional liquor burner capableof processing 36 m³/hr of LTD (liquor to digestion) is shown in Table 1.This may be compared with the performance of the proposed combinedsystem (Table 2), in which the same liquor burner is linked with a wetoxidation system operating at 175° C. and an oxygen partial pressure of4 atmospheres. The LTD has an organic carbon content of 20 g/L.

TABLE 1 Estimate of TOC destruction performance of conventional LiquorBurner LTD flow to Liquor Burner 36 m³/hr LTD flow to leach tank 322m³/hr Total LTD Flow 358 m³/hr Total TOC input 7.2 t/hr TOC destroyed byLiquor Burner 0.72 t/hr % TOC destroyed 10.0%

TABLE 2 Estimate of TOC destruction performance of Combined System LTDflow to Liquor Burner 5 m³/hr LTD flow to Wet Oxidiser 353 m³/hr TotalLTD Flow 358 m³/hr Total TOC input 7.2 t/hr TOC destroyed by combinedsystem 1.44 t/hr % TOC destroyed 20.0% % Improvement in TOC removalrelative to Liquor 100% Burner only

It can be seen that the combined system is capable of destroying andfully causticising twice the amount of TOC as the liquor burner alone.However, the performance improvement offered by the disclosed process isnot limited to the example shown above, and both higher and lowerconversion efficiencies can be achieved depending upon the configurationand size of the wet oxidation and liquor burning units. Ideally, theflow of liquor to the wet oxidation unit is sized such that the amountof sodium carbonate and sodium oxalate solids that precipitate in theleach tank precisely matches the input requirements of the liquorburning unit, with no excess. However, smaller wet oxidation units canbe used (with correspondingly smaller improvements in TOC removalcapacity), simply by adjusting the direct feed of LTD to the liquorburner.

The operation of the leach tank may be critical to the performance ofthe process. If the caustic concentration of the leach tank is too high,entrainment of liquor within the precipitated solids cake becomessignificant. This entrained liquor, being of high concentration, resultsin the recycle of excessive amounts of sodium aluminate and sodiumhydroxide to the liquor burner, reducing efficiency. On the other hand,too low a concentration may result in too little sodium carbonate andsodium oxalate precipitating. The exact concentration at which tooperate the leach tank will depend upon many factors including theconfiguration of the wet oxidation and liquor burning units, and thecomposition of the feed (LTD) liquor. A balance is sought between thedissolution of the sodium aluminate in the leach liquor and theprecipitation of sodium carbonate, sodium oxalate and any organic sodiumsalts which may also precipitate, such that the amount of sodium saltsprecipitated matches the requirements of the liquor burner orsalting-out kiln. Furthermore, in most instances, sodium sulphateprecipitation must also be minimised, unless specific steps are takenfor its simultaneous removal. In most instances, however, bestperformance will be obtained at a ‘C’ concentration in the leach tank ofapproximately 300 g/L.

EXAMPLE 2

FIG. 2 illustrates a variation of the process of FIG. 1, in which thelike parts have been identified with the same reference numerals. Thismodified process requires a larger wet oxidation facility to achieve asimilar organic removal capacity but reduces the emission of volatileorganic carbon (VOC) compounds from the liquor burner 16. The processvaries from the basic concept illustrated in FIG. 1, in that all of theorganics-rich spent liquor 10 is fed to the wet oxidation process 12,and then the processed liquor from the wet oxidation process 12 is usedto feed the leach tank 18. There is no direct flow of liquor to the mixtank 14 of the liquor burner 16. In this way, the input to the liquorburner 16 is greatly reduced in organic content, as most has beenconverted to sodium carbonate and sodium oxalate. The reduction inorganic carbon to the liquor burner 16, especially of humic materials,can result in substantial improvements in the odour and VOC content ofthe liquor burner's stack gas emissions. No change in the size of theliquor burning unit is required.

The direct flow of liquor to the mix tank 14 has been omitted becausethe filtered solids from the leach tank 18 generally contain sufficiententrained liquor to ensure good pellet formation within the liquorburner's drying and pelletising unit. Consequently, the cake, togetherwith the appropriate amount of alumina or gibbsite, can be dispersed inthe mixing tank 14 with just sufficient water (if required) to produce auniform paste. Removing the direct feed of liquor to the liquor burner16 improves the TOC removal capacity of the process by minimising theunproductive input of sodium aluminate and sodium hydroxide.

In both of the configurations illustrated in FIGS. 1 and 2, additionalcapacity can be achieved by incorporating a suitable evaporator prior tothe wet oxidation process 12. This will provide the wet oxidation unitwith a stream that is enriched with organics, enhancing performance.Deep evaporators such as those used in the prior art liquor burning orsalting-out evaporation processes are not required. Indeed, any one ofthe refinery's existing evaporators would be suitable for the purpose.In this instance, heat from the wet oxidation process can be recoveredfor use by the evaporator.

EXAMPLE 3

A disadvantage of the two preceding configurations arises when the Bayerprocess stream contains high concentrations of sodium sulphate. Sodiumsulphate does not react with alumina in the liquor burner, and passesthrough the system intact. Owing to its comparatively low solubility, itwill precipitate in the leach tank and will thus recycle with the otherprecipitates. The recirculating load will quickly rise to unacceptablelevels, and the process will fail to operate. This disadvantage isovercome in the preferred embodiment of the process illustrated in FIG.3.

The preferred embodiment of the process for removal of organicimpurities from a Bayer process liquor as illustrated in FIG. 3 issimilar to that of FIG. 2, except that it is combined with the sulphateremoval process disclosed in commonly-owned Australian Patent No.673306. The disclosure of Australian Patent No. 673306 is incorporatedherein by reference. Once again, the like parts in the process of FIG. 3have been identified with the same reference numerals as in FIGS. 1 and2. An optional evaporator 28 has been incorporated prior to the wetoxidation unit 12. The process of the invention as embodied in FIG. 3may be combined with the sulphate removal process of 673306 by theaddition of an additional leach tank 22 and solid/liquid separation unit24. A splitter is used to direct a proportion of the processed dischargeproduct from the liquor burner 16 into a “dirty” leach tank 18, which isfed with processed liquor from the wet oxidation process 12. Solidsprecipitating from this tank 18 are recycled to the liquor burner mixtank 14, as in the preceding embodiments.

The remaining portion of the liquor burner kiln discharge product isdirected into a “clean” leach tank 22 fed with a caustic solution ofsufficient concentration to ensure that the solubility of gibbsite isnot exceeded. Alternatively, a suitable process liquor stream can beused. The feed of the processed discharge product from the liquor burner16 to the leach tank 22 is controlled such that the amount of sodiumsulphate in the discharge product is equal to the total input ofsulphate to the process. The high caustic and low carbonateconcentration of the liquor in the “clean” leach tank 22 ensures thatonly sodium sulphate is precipitated. The slurry from tank 22 isfiltered in the solid/liquid separation unit 24, and the solids leachedin water in leach tank 26. The resultant sodium sulphate solution isthen disposed of or purified, as in Australian Patent No. 673306, andthe remaining alumina solids are recycled to the liquor burner mix tank14.

TEST RESULTS

Typical unit operations of the proposed process were simulatedindividually in the laboratory. Products from each of these operationswere then passed on to the next operation in the sequence, such that afull model simulation of the process was finally obtained. Forcomparison, the operation of a conventional liquor burning unit was alsosimulated. Results reported here are based on this comparison.

Wet Oxidation

Spent liquor (LTD) from a Western Australian refinery was used as thefeedstock throughout the test programme. This liquor is typical of manyalumina refineries. Five litres of this liquor was added to a 17 litreautoclave (Parr Instrument Company, Illinois, USA), and oxygen added toachieve a partial pressure of 20 atmospheres. The mixture was thenheated to 175° C. without agitation. Once at temperature, agitation wascommenced, using twin pitch-blade turbine impellors operating at 700rpm, for a period of 30 minutes. The reaction temperature was maintainedat 175° C. through a combination of the reactor's heating elements and aserpentine cooling coil. After 30 minutes had elapsed, the mixture wasrapidly cooled to 85° C. using the cooling coil and the resultant slurryfiltered through a Supor membrane (0.45 μm) filter. The solids weredried at 105° C.

While a higher temperature will increase the efficiency and capacity ofthe process, 175° C. was chosen primarily because very little hydrogenis evolved at this temperature (<0.3% in the off-gases). In addition,this temperature is easily accessible even in low temperature aluminarefineries: the process can be fed from the heater side of one of therefinery's existing evaporators, with additional temperature riseprovided by the exothermic reaction of the organics. A typical analysisof the liquor before and after the wet oxidation procedure is shown inTable 3 below.

TABLE 3 Typical Results of Wet Oxidation Procedure A C S Na₂CO₃ Na₂C₂O₄TOC Density Liquor g/L g/L g/L A/C C/S g/L g/L g/L g/mL Untreated LTD105.4 234.2 292.7 0.45 0.800 58.5 4.3 27.4 1.365 Wet Oxidised 104.7206.5 289.0 0.507 0.714 82.53 4.87 23.0 1.363 LTD

Leach Tank

Approximately 8500 mL of wet oxidised liquor (collected from two wetoxidation runs) was transferred to an agitated stainless steel vessel,heated to 95° C. A proportionate amount of the solids that precipitatedas a result of the wet oxidation procedure were added to this liquid,together with 1350 g of pure sodium aluminate (Sumitomo Chemical,Japan). This latter material was intended to simulate the addition ofkiln product. The amount of sodium aluminate was determined by thetarget ‘C’ concentration in the leach tank of 300 g/L. After addition ofthese solids, the mixture was permitted to equilibrate for 60 minutes,before filtration. This filtration was performed at 95° C. and under apressure of 400 kPa. These conditions were selected so as to minimiseentrained liquor in the filter cake. Analyses of the liquid and solidsobtained from this process are shown in Tables 4 and 5 respectively.Solids analyses were performed by a combination of dry (XRF) and wet(TOC, TIC) analytical techniques on samples that had been dried toconstant weight. TIC refers to the total inorganic carbon content.

TABLE 4 Typical Liquor Analysis After Leaching and Precipitation A C SNa₂CO₃ Na₂C₂O₄ TOC Density g/L g/L g/L A/C C/S g/L g/L g/L g/mL 189.2312.5 381.5 0.606 0.819 69.0 2.9 22.4 1.443

TABLE 5 Typical Analysis of Precipitated Solids Na₂O Al₂O₃ SO₃ NaCl LOITIC TOC Total % % % % % % % % 49.7 10.4 22.1 0.8 12.1 3.6 2.5 101.2

Pelletiser (Paddle Mixer)

The wet cake was mixed with gibbsite from a Western Australian refineryand additional water added, to form a thick slurry. The ratio ofgibbsite to wet oxidised solids was chosen to give a molar ratio ofAl₂O₃ to Na₂O of approximately 1:1. This slurry was charged into acustom made cylindrical stainless steel pelletiser (length 610 mm,diameter 220 mm). The pelletiser, equipped with a stainless steel rod(19 mm diameter) and an internal thermocouple, was kept under partialvacuum and rotated at 25 rpm inside a muffle furnace, the insidetemperature being kept at 150-170° C. Pelletisation was consideredcomplete when no further water was released from the solids.

A similar procedure was followed to simulate pellet formation for aconventional liquor burner, using untreated LTD. Typical analyses forthe pellets produced for both the proposed process and for aconventional liquor burner are shown in Table 6.

TABLE 6 Typical analysis of simulated pelletiser product Na₂O Al₂O₃ SO₃NaCl LOI TIC TOC Total % % % % % % % % New 28.5 36.9 9.5 0.8 23.3 1.61.5 102.2 Process Liquor 28.4 42.4 3.3 2.4 24.6 0.8 2.4 104.5 Burner

Dryer and Kiln

The effect of the preferred process on volatile organic carbon (VOC)emissions was examined using pellets prepared in the previous step. Theconditions that result in the evolution of VOC's in a liquor burner'sdrying circuit and kiln were simulated in the laboratory using a musefurnace operating at 950° C., through which dry clean compressed air wasflowed at a rate of 10 normal litres per minute. Approximately 20 g ofpellets from either the improved process or conventional liquor burnerwas weighed into a vitreous silica crucible and the crucible placed inthe pre-heated oven. Air passing through the oven was sampled at a rateof 4 normal litres per minute.

The sampled air was passed through a two-stage trap to collect evolvedVOC's. The first trap consisted of a stainless steel coil cooled to 0°C. and a Dreschel bottle cooled to −12° C. within which water andwater-soluble organics were collected. The second trap consisted of astainless steel vessel filled with borosilicate glass Raschig rings,cooled with liquid nitrogen, within which the remainder of the VOC'swere collected.

This evolution and sampling procedure was repeated until 87.33 g of eachof the pellet types had been processed and the evolved organicscollected.

Analysis of the collected organics was performed by solid phasemicro-extraction of the headspace above the relevant samples followed byGC/MS analysis. Comparison of the total evolved organics was carried outon the basis of the total integrated peak areas. While thesemeasurements do not directly give the total mass of VOC's evolved, itpermits a simple comparison of the VOC emissions from the improvedprocess relative to the conventional liquor burning process. Typicalresults are shown in Table 7, in which the integrated peak areas havebeen normalised against that of the conventional or burning process.

TABLE 7 Relative VOC emissions Process Normalised Integrated Peak AreaNew Process 11.75% Conventional Liquor Burner 100.0%

It can be seen from Table 7 that in this example the use of the improvedprocess has resulted in an approximate 9-fold reduction in VOC emissionsrelative to the conventional liquor burning process. The extent of theimprovement in VOC emissions achieved in practice will be dependent uponthe configuration of the wet oxidation and liquor burning units, and theconcentration and character of the organics within the feed liquor.

Chromatograms for VOC's collected from the Improved Process wereconsiderably simpler (far fewer peaks) than those of the conventionalliquor burner. Identification of some of the more abundant components ofthe evolved VOC's was performed using selective ion mass spectrometry.Typical results are shown in Table 8.

TABLE 8 Comparison of specific evolved VOC's % of total VOC's multiplied% of total VOC's by normalised peak area Conventional ConventionalImproved Liquor Improved Liquor Species Process Burner Process BurnerBenzene 0 0.49% 0 0.49% Naphthalene 7.6% 6.9% 0.89% 6.9% Toluene 0 0.06%0 0.06% 1-methyl-3-phenoxy 1.83% 0.04% 0.22% 0.04% benzene Benzaldehyde0 0.23% 0 0.23% 1-methyl-4- 0.27% 3.65% 0.03% 3.65% nitrobenzene Benzeneacetonitrile 0.11% 0.26% 0.01% 0.26% Biphenyl 6.9% 10.5% 0.81% 10.5%Cyclohexadiene 11.6% 3.5% 1.4% 3.5% derivative

The first set of columns in Table 8 represent the percentage of eachspecies in the off-gases from each of the two processes. In some cases,the percentage of a particular species has increased, however, since thetotal mass of VOC's evolved by the improved process is substantiallysmaller, the evolved mass of these species is invariably lower than fora conventional liquor burner. This is shown in the second set ofcolumns. The above results show that, along with a substantial reductionin the total amount of each species (relative to a conventional liquorburner), the nature of the organics evolved is also markedly different,with the improved process producing far less aromatic and polyaromaticcompounds.

The calcined product remaining in the crucible was analyzed by acombination of dry (XRF) and wet (TOC, TIC) techniques. Typical analysesobtained for the calcined product are shown in Table 9.

TABLE 9 Analysis of Calcined Product Na₂O Al₂O₃ SO₃ NaCl TIC TOC Total %% % % % % % New 37.1 41.1 16.2 0.86 0.53 0.06 95.9 Process Liquor 37.452.4 3.85 2.87 0.22 0.11 96.9 Burner

Results and Comparison of Proposed Process and Prior Art

The following results and comparisons arc based on the premise that theamount of sodium aluminate produced by the liquor burner is heldconstant. Consequently, the results are presented on the basis of TOCdestruction per kilogram of “reactive soda” entering the kiln. “Reactivesoda” is defined as the sodium content of the liquor or pellets that iscapable of reacting with alumina within the kiln to form sodiumaluminate, expressed as sodium carbonate. Thus, sodium sulphate andsodium chloride, which do not normally react in this way, are notincluded as reactive soda.

In practice, this form of comparison represents the minimum improvementpossible, because the reduced water input to the liquor burner affordedby the new process will permit greater throughput of solids through thekiln.

TABLE 10 Performance of Simulated Conventional Liquor Burner InputsLiquor (LTD) in: 3150.2 mL TOC in: 86.22 g Pellets produced: 2504 gReactive soda in 1049.4 g pellets: NaAlO₂ in: 1141.6 g Al(OH)₃ added:1129 g Outputs NaAlO₂ out: 1623.6 g TOC out: 3.95 g Performance (per kgReactive Soda) “New” NaAlO₂ 482.0 g produced: TOC destroyed 78.4 g

TABLE 11 Performance of Proposed Combined Process Inputs Liquor (LTD)in: 7978.9 mL TOC in: 218.4 g Pellets produced: 1515.0 g Reactive sodain 353.1 g pellets: NaAlO₂ in: 185.9 g Al(OH)₃ added: 855.0 g OutputsNaAlO₂ out: 546.4 g TOC out (liquor): 167.9 g TOC out (pellets) 0.69 gPerformance (per kg Reactive Soda) “New” NaAlO₂ 1021.0 g produced: TOCdestroyed 141.1 g

TABLE 12 Performance Comparison Conventional Parameter Liquor % (per kgReactive soda) Burner New Process improvement TOC destruction 78.4 g141.1 g 80.0% “New” NaAlO₂ production 459.3 g 1021.0 g 122.3% Percent“new” NaAlO₂ 29.7% 66.0%

The theoretical improvement in performance for the above laboratory testwas 99%, relative to a conventional liquor burner. The improvementobtained (80%) is less than the theoretical maximum primarily because ofentrained liquor in the precipitate from the leach tank. This entrainedliquor reduces the capacity of the liquor burner by recycling sodiumaluminate and sodium hydroxide, neither of which contributes to the TOCremoval capability of the liquor burner.

The “new” NaAlO₂ production figure reported above refers to sodiumaluminate not already contained in the feed liquor to the process (forthe purpose of this calculation, sodium hydroxide is also considered assodium aluminate). This is a measure of the causticising efficiency ofthe process, and like the TOC removal performance, is strongly affectedby the recycle of entrained liquor with the leach tank precipitate. Itshould be noted that while much of the “new” NaAlO₂ derives from sodaassociated with the additional TOC destroyed, some comes from sodiumcarbonate already present in the feed liquor This highlights anotheradvantage of the proposed process: the removal and causticisation ofadditional sodium carbonate from the refinery's liquor streams.

The effect of entrained liquor on the performance of the process isdemonstrated by the results shown in Tables 13 and 14 below.

TABLE 13 Effect of Entrained Liquor on Performance of Proposed ProcessInputs Liquor (LTD) in: 7842.8 mL TOC in: 214.7 g Pellets produced: 1263g Reactive soda in 447.7 g pellets: NaAlO₂ in: 298.1 g Al(OH)₃ added:539.5 g Outputs NaAlO₂ out: 692.7 g TOC out (liquor): 164.9 g TOC out(pellets) 0.90 g Performance (per kg Reactive Soda) “New” NaAlO₂ 881.4 gproduced: TOC destroyed 109.2 g

TABLE 14 Performance Comparison Conventional Parameter Liquor % (per kgReactive soda) Burner New Process improvement TOC destruction 78.4 g109.2 g 39.3% “New” NaAlO₂ production 459.3 g 881.4 g 91.9% Percent“new” NaAlO₂ 29.7% 57.0%

Clearly, best performance is obtained by ensuring that the recycledprecipitate from the leach tank is thoroughly deliquored. This isassisted by ensuring that the slurry being filtered is kept as hot aspossible, to reduce the viscosity, and by targeting ‘C’ concentrationsin the leach tank as low as is practicable (a suitable value isapproximately 300 g/L).

From the above description of preferred embodiments of the organicimpurity process for Bayer liquors, a number of advantages of thecombined system will be apparent, including the following:

(i) Wet oxidation is a simple organic removal process, but it suffersfrom the serious disadvantage of generating sodium carbonate and sodiumoxalate as reaction products. These must be either further treated ordiscarded. On the other hand, liquor calcination techniques such asLiquor Burning produce a product that can be directly utilised in thealumina refinery, but are very energy inefficient. Most of this energyis expended in reducing the Bayer liquor to dryness, and only a smallproportion of the dissolved solids in the liquor is actively involved inthe organic removal process. The proposed process overcomes both ofthese limitations by combining the two processes in such a way that theweakness of each individual process becomes a strength of the combinedprocess. When combined in the manner proposed, the TOC destructioncapacity of an existing liquor burner or other liquor calcinationprocess can be increased by 80% or more.

(ii) In addition to TOC removal, the proposed process can be used toincrease the causticity (C/S) of the refinery's liquor streams. This isachieved by adjusting the conditions of the leach tank so that, inaddition to the products of wet oxidation, much of the sodium carbonatealready present in the feed liquor is precipitated and fed to the liquorburner.

(iii) The deep evaporator commonly used to concentrate the feed streamto a liquor burner is costly in energy and prone to operating problemssuch as fouling. With appropriate sizing of the wet oxidation unit andleach tank, the proposed process allows the deep evaporator to beeliminated entirely.

(iv) If a deep evaporator is used, the low humate concentration of thewet oxidised liquor feeding the liquor burner will reduce the viscosityand surface tension of the liquor, improving the performance of the deepevaporator.

(v) Heat from the wet oxidation process can be recovered for useelsewhere, reducing energy consumption.

(vi) The low humate concentration of the feed to the leach tank assistsin the crystallisation of solids in the leach tank (sulphate, carbonateand oxalate). The resultant leach slurry is lower in viscosity than anequivalent liquor using untreated spent liquor, improving solid/liquidseparation.

(vii) By feeding the liquor burner primarily with inorganic solids, thepotential for odour and VOC emissions from the liquor burner is greatlyreduced. This can reduce or eliminate the need for treatment of thestack gas emissions of the liquor burner using afterburners or similarVOC destruction technology.

Numerous variations and modifications to the described process, inaddition to those already described, will suggest themselves to personsskilled in the chemical engineering arts, without departing from thebasic inventive concepts. For example, the process of the invention maybe combined with any suitable sulphate removal process In order tocontrol the recirculating load of sodium sulphate. All such variationsand modifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription and the appended claims.

The claims defining the invention are as follows:
 1. A process for the removal of organic impurities from a Bayer process liquor, comprising: feeding a first liquor stream including organic impurities to a wet oxidation process to produce a first processed liquor which is depleted in organic compounds, but enriched with sodium carbonate and/or sodium oxalate; reacting sodium compounds with at least one of an aluminum oxide, an aluminum hydroxide, an iron oxide and an iron hydroxide in a feed slurry fed to a dry oxidation process, wherein at least part of the sodium compounds react in the dry oxidation process to produce a processed discharge product; feeding at least a first portion of the first processed liquor to a leach tank to which is added the processed discharge product from the dry oxidation process, wherein the sodium carbonate and/or sodium oxalate precipitate in the leach tank to form precipitated products; and separating the precipitated products from the leach tank and recycling the precipitated products to form at least part of the feed slurry fed to the dry oxidation process; wherein organic impurities in the first liquor stream and residual organic impurities remaining in the first processed liquor or in the recycled precipitated products are causticised to sodium aluminate or sodium ferrate in the dry oxidation process.
 2. A process for the removal of organic impurities as defined in claim 1, wherein a second portion of the first processed liquor is mixed with the recycled precipitated products forming at least part of the feed slurry fed to the dry oxidation process.
 3. A process for the removal of organic impurities as defined in claim 1, wherein all of the first processed liquor is fed to the leach tank.
 4. A process for the removal of organic impurities as defined in claim 1, further comprising: splitting a Bayer liquor stream into the first liquor stream fed to the wet oxidation process and a second liquor stream; and combining the second liquor stream with at least the recycled precipitated products to form the feed slurry.
 5. A process for the removal of organic impurities as defined in claim 1, wherein the dry oxidation process employs a liquor burner.
 6. A process for the removal of organic impurities as defined in claim 5, wherein the wet oxidation process includes an evaporator.
 7. A process for the removal of organic impurities as defined in claim 5, further comprising: removing sulphate from the processed discharge product.
 8. A process for the removal of organic impurities as defined in claim 7, wherein a proportion of the processed discharge product from the dry oxidation process is fed to a second leach tank liquor including a caustic concentration sufficient to ensure gibbsite solubility is not exceeded, and wherein the feed of the proportion of the processed discharge product to the second leach tank liquor is regulated to ensure that an amount of sodium sulphate in the processed discharge product is substantially equal to a total input of sulphate to the process.
 9. A process for the removal of organic impurities as defined in claim 8, further comprising: ensuring that a selected caustic to soda ratio is maintained in the second leach tank liquor such that substantially only sodium sulphate is precipitated; and separating the precipitated sodium sulphate from the second leach tank liquor.
 10. A process for the removal of organic impurities as defined in claim 9, further comprising: leaching the precipitated sodium sulphate in a third leach tank liquor to form a sodium sulphate solution; separating alumina solids from the sodium sulphate solution; and feeding the alumina solids to a mix tank, wherein the alumina solids are combined with at least the recycled precipitated products in the mix tank to form the feed slurry. 